McCarthy was a key player for the Latics during last season's successful Premier League survival campaign and is reportedly attracting interest from other top-flight clubs.
However, Martinez revealed positive talks are "ongoing" with the Republic of Ireland international and called for calm amongst fans fearing the player could make an exit.
"We have been working really hard to give the players the rewards they have deserved, and as you can imagine James is one of those players," he told the Wigan Observer.
"I can tell you that negotiations have not broken down at all - these things just take time.
"But the talks are very much ongoing, and there will be a successful resolution. It's not as if he is in his last year - he already has a long-term contract here.
"It's more of a reward for the player than anything else. But these things come out in the national press, and sometimes you just can't reason why that is the case.
"For the moment, though, James is focusing on his football, and we'll let the agents focus on the financial side of things."
Source: Team Talk
Source: Team Talk